JP 2001-297748 A discloses a nonaqueous electrolyte secondary battery comprising a flat battery element, a bag-like exterior case which houses the flat battery element, a positive electrode lead which is led out from a sealing part of the exterior case, and a negative electrode lead which is led out from the sealing part of the exterior case. The flat battery element is configured such that sheet-like or film-like positive electrode plates, separators for holding electrolyte, and negative electrode plates are laminated. One end of the positive electrode lead is connected to the positive electrode plates and one end of the negative electrode lead is connected to the negative electrode plates.
The above secondary battery has an anisotropic property in the contraction ratio. That is, when the charge and discharge are repeated and accordingly the electrodes expand in the thickness direction, the contraction ratio is different between in the direction along the long side and in the direction along the short side with respect to the lamination surface of the electrodes. The above secondary battery has one extra space and the other extra space, the one extra space is formed at the short side of the lamination surface of the electrodes, the other extra space is formed at the long side of the lamination surface of the electrodes, and the one extra space and the other extra space are formed between the laminate of electrodes and the laminated exterior case. In addition, when the electrodes expand in the thickness direction, the volume of the one extra space is excessively larger than the volume of the other extra space. Therefore, the electrolyte solution and the generated gas concentrate in the other extra space to lead to un-uniform cell reaction and poor durability of the cell, which may be problematic.